Ongoing studies in this project concern the basic cell biology, biochemistry and molecular biology of Leishmania, a group of protozoan pathogens of humans. All Leishmania parasites undergo a cyclic developmental cycle: 1) in mammals, they transform into and multiply as obligate intracellular [amastigote] forms within the phago-lysosomal system of host macrophages and 2) in their hematophagous sandfly-vector hosts, they differentiate into, and multiply as, extracellular [promastigote] forms within the insect's alimentary tract. Various species of this organism cause over 12 million cases of human disease worldwide. In infected humans, these parasites destroy macrophages within the skin or internal organs (spleen, liver and bone marrow) causing either disfiguring ulcerative cutaneous lesions (e.g., caused by L. mexicana) or degenerative and fatal visceral disease (e.g., caused by L. donovani). Previous studies from our laboratory have established that Leishmania parasites are apically polarized secretory cells and that they constitutively secrete over 40 different soluble protein, glycoprotein and carbohydrate constituents into their growth media in vitro. Such soluble, extracellularly secreted products can readily diffuse away from these parasites and permeate their environments. In nature, these organisms reside and multiply within various aqueous micro environments in their several hosts. Since these organisms actively secrete a variety of different enzymes that could alter such host micro- environments, an understanding of the nature of these parasite enzymes seems essential. To that end, several parasite secretory enzymes are investigated toward defining their functional roles in the survival, maintenance, growth and transmission of these organisms. Further, genes encoding these proteins are being identified and characterized toward defining their expression and regulation during parasite growth, development and differentiation. Experiments involving both the in situ deletion and over-expression of such genes should demonstrate whether their encoded proteins are, in fact, essential to this parasite's survival.